This application claims the priority of Korean Patent Application No. 2003-0004579, filed on Jan. 23, 2003, in the Korean Intellectual Property Office, the contents of which are incorporated herein in their entirety by reference.
1. Field of the Invention
The present invention relates to a method and apparatus by which the position of a position estimation apparatus is estimated using a plurality of satellite signals that are output with predetermined time differences from one or more global positioning system (GPS) satellites.
2. Description of the Related Art
Generally, position estimation methods using the GPS satellite system estimate a range between the GPS satellite and the antenna of a position estimation apparatus (for example, a GPS receiver), using triangulation and clean and acquisition (C/A, or coarse and acquisition) code which is output from the GPS satellite.
The GPS satellite always transmits an L1 frequency of 1575.42 MHz that carries a C/A code. A position estimation apparatus generates the same code as the C/A code. The generated C/A code is compared with the received C/A code of the GPS satellite. From the result of the comparison, the time it takes for the C/A code output from the GPS satellite to arrive at the position estimation apparatus is measured.
The position estimation apparatus measures the range between the GPS satellite and the position estimation apparatus by using the result of multiplication of the speed of light (the speed of the C/A code output from the GPS satellite) and the time taken for arrival. Since the C/A code comprises a pseudo random noise code that is almost noise in itself, and the measured range between the GPS satellite and the position estimation apparatus contains errors, the range is referred to as a pseudo range.
A conventional position estimation apparatus receives satellite signals simultaneously output from at least four GPS satellites, measures the pseudo range corresponding to each satellite signal, and from the measured results estimates the position of the position estimation apparatus. That is, the position estimation apparatus estimates a 3-dimensional position of the position estimation apparatus by simultaneously using four or more satellite signals output from four or more GPS satellites.
However, due to the influence of the environment, the position estimation apparatus at times cannot receive at least four signals at the same time. In this case, the conventional position estimation apparatus cannot estimate its 3-dimensional position.